1. Field of the Invention
The present invention relates to a so-called gas turbine combined plant or a combined-cycle plant and a method of operating the same. The present invention further relates to a steam-cooling system for a hot section in a gas turbine in which the hot section such as a combustor in the gas turbine of the combined-cycle plant may be cooled by steam from an exhaust heat recovery boiler as an example.
2. Description of the Related Art
Hitherto, air has been utilized to cool hot sections or elements such as combustors in gas turbines. However, with a need for higher combustion temperature levels, there has been developed a method of cooling these elements by steam. Furthermore, a plan has been undertaken in which gas turbines containing the hot sections to be cooled by steam are applied to a combined-cycle plant and combined with steam turbines, thereby providing a high efficient power plant.
FIG. 2 is a schematic view showing an example of a gas turbine steam system in conventional combined-cycle plants. In FIG. 2, an exhaust gas from a gas turbine 1 is delivered into an exhaust heat recovery boiler 2. The steam from a high-pressure drum 5 in the exhaust heat recovery boiler 2 is delivered into a high-pressure steam turbine 3 through a high-pressure superheater 6 and a high-pressure steam governing valve 13. On the other hand, the steam from an intermediate-pressure drum 9 in the exhaust heat recovery boiler 2 is delivered into an intermediate-pressure steam turbine 4 through an intermediate-pressure superheater 10, a reheater 7, and an intermediate-pressure steam governing valve 14 in the order named.
The steam is then supplied to a hot section 8 from a cooling steam source 12, and is used to cool the hot section 8. Auxiliary steam of the power plant or the steam (for example, the steam at the outlet of the intermediate-pressure superheater 10) in the exhaust heat recovery boiler 2 may be used as the cooling steam source 12. The steam after cooling is then confluent with the steam at the outlet of the reheater 7 to be fed together to the intermediate-pressure steam turbine 4 through the intermediate-pressure steam governing valve 14, as described above.
Such a combined-cycle plant is provided with a system having the following arrangement. During starting up the plant or at the start-up time of the gas turbine unit 1, the steam temperature detected at a hot section outlet steam temperature detector 15 and the steam temperature detected by a reheater outlet steam temperature detector 16 are both too low to allow the steam to be effectively directed to the intermediate-pressure steam turbine 4. Thus, an intermediate-pressure steam turbine inlet steam temperature detector 22 is provided to a pipe connected to the intermediate-pressure steam governing valve 14, and detects the steam temperature thereof. While the intermediate-pressure steam governing valve 14 is held closed based upon this detection signal, the steam can by-pass from a turbine by-pass valve 18 to a condenser 11. When the steam temperature rises to reach a tolerance, the intermediate-pressure steam governing valve 14 is then opened based upon the detection signal from the intermediate-pressure steam turbine inlet steam temperature detector 22, resulting in starting up the intermediate-pressure steam turbine 4.
However, while the temperature rises as described above, it has been found that, in a certain period, the temperature detected by the hot section outlet steam temperature detector 15 may be kept considerably higher in rising ratio than the temperature detected by the reheater outlet steam temperature detector 16. This fact indicates that there occurs a large difference between both the outlet steam temperature levels. Accordingly, there is a fear that an undesirable thermal stress be generated at the point, or in the vicinity thereof, where the pipe extending from the reheater 7 and the pipe extending from the hot section 8 are confluent with each other.